The program is aimed at extending our knowledge of the factors regulating the rates of synthesis and turnover of soluble, cytoplasmic proteins in mammalian tissues, using rat liver ferritin as a model system. This is a particularly favorable model since we have devised biochemical and immunological procedures that enable us not only to quantitate the intracellular amounts of this protein but also to follow its rate of synthesis and degradation by tracer studies with Fe 59 and C 14 amino acids. Administration of iron causes an increase in the ferritin level which may be attributable both to increased de novo synthesis and to retarded breakdown of preformed ferritin. This model system, therefore, provides a situation susceptible to detailed exploration of the mechanisms regulating mammalian protein synthesis and turnover. Evidence points to regulation at the level of translation; attempts to seek factors controlling ferritin synthesis in a cell-free system will be coupled with efforts to determine the mode and site of its degradation within the cell. Structural and functional relationships of the organ-specific isoferritins recently displayed by gel electrofocusing will be investigated to assess the physiological importance of these isoferritins and their significance for the metabolic studies.